This application is based upon and claims the benefit of Japanese Patent Application No. 2000-79345 filed on Mar. 16, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a semiconductor device having a resistive element made of semiconductor.
2. Description of the Related Arts
Conventionally used semiconductor resistors (resistive elements) are a diffusion resistor, a poly silicon resistor, and so forth. A diffusion resistor is formed, for example, by doping impurities into a semiconductor substrate by ion implantation, and then by thermally diffusing the impurities. A poly silicon resistor is formed, for example, by forming a poly silicon film into which impurities are doped through an insulating film on a semiconductor substrate, and by patterning the poly silicon film.
FIG. 10 schematically shows a diffusion resistor formed in a semiconductor substrate. The diffusion resistor has a value of resistance that is determined by resistance length (distance between both ends) L and sectional area S thereof. To increase the value of resistance, at least one of width T and thickness D of the diffusion resistor is decreased so that the sectional area S is decreased, or the resistance length L is lengthened. As opposed to this, to decrease the value of resistance, at least one of the width T and the thickness D of the diffusion resistor is increased, or the resistance length L is shortened.
When the width T and the thickness D are decreased to increase the value of resistance, however, a current density in the diffusion resistor is increased. The increased current density is liable to cause damages to the element. Further, the lengthened resistance length L is disadvantageous to high integration. The increased with T of the diffusion resistor is also disadvantageous to high integration. The increased thickness D requires larger number of times of ion-implantations, and a longer period of heat treatment time, thereby lowering throughput. The lengthened resistance length L further causes an increase of variation thereof.
The present invention has been made in view of the above problems. An object of the invention is to provide a semiconductor device including a resistor structure with high resistance and high reliability. Another object of the invention is to provide a semiconductor device including a resistor structure suitable to high integration even when a resistance length is increased.
According to the present invention, briefly, a semiconductor device has a semiconductor substrate of a first conductivity type, and a resistive element of a second conductivity type, extending from a main surface in a thickness direction of the semiconductor substrate. The resistive element has an impurity concentration that is approximately uniform in the thickness direction of the semiconductor substrate.
In the semiconductor device, for example, when a resistance length of the resistive element is parallel to the thickness direction of the substrate, the resistance length can be increased by increasing the depth of the resistive element, resulting in high resistance of the resistive element. Even when the resistance length is increased as above, the increased resistance length does not disturb high integration of the element.